At present, a catalyst of a honeycomb structure (a honeycomb catalyst) has been used in purifying exhaust gases discharged from various engines and the like. As shown in FIG. 6, this honeycomb catalyst has a structure in which a catalytic layer 15 is loaded on the surface of a partition wall 4 constituting a cell 3. Further, as shown in FIGS. 4 and 5, to purify the exhaust gas by use of this honeycomb catalyst (a honeycomb structure 11), the exhaust gas is flowed into the cells 3 of the honeycomb catalyst from one end surface 2a side, brought into contact with a catalytic layer (not shown) on the surfaces of the partition walls 4, and then discharged to the outside from the other end surface 2b side (see, for example, Patent Document 1).
When such a honeycomb catalyst is used to purify the exhaust gas, transmission of unpurified components contained in the exhaust gas from the exhaust gas toward the catalytic layer on the surface of the partition wall must be promoted as much as possible to improve a purification efficiency. In order to improve the purification efficiency of the exhaust gas, for example, a hydraulic diameter of the cell must be reduced, and a surface area of the partition wall must be increased. Specifically, for example, a method of increasing the number of cells per unit area (a cell density) is adopted.
Here, it is known that a transmission rate of unpurified components from the exhaust gas toward the catalytic layer on the surface of the partition wall is increased in inverse proportion to a square of the hydraulic diameter of the cell. Therefore, the transmission rate of the unpurified components is improved as the cell density is increased. However, a pressure loss also tends to increase in inverse proportion to a square of the hydraulic diameter. Accordingly, there is a problem that the pressure loss is increased with an improvement in the transmission rate of the unpurified components.
Incidentally, a thickness of the catalytic layer on the surface of the partition wall is usually approximately several-ten μm. Here, if a spreading speed of the unpurified components in the catalytic layer is insufficient, a purification efficiency of the honeycomb catalyst tends to be lowered. This tendency is considerable especially under low-temperature conditions. Therefore, in order to increase the purification efficiency for the exhaust gas, it is necessary not only to increase the surface area of the catalytic layer but also to reduce the thickness of the catalytic layer to improve a spreading speed of the unpurified components in the catalytic layer. Accordingly, there is an advantage that the surface area of the catalytic layer becomes large when the cell density is increased and, on the other hand, there is a problem that the pressure loss is increased.
In order to reduce the pressure loss while increasing the purification efficiency for the exhaust gas, an inflow diameter of the honeycomb catalyst must be enlarged and a flow rate of the circulated exhaust gas must be lowered. However, when the honeycomb catalyst is grown in size, for example, a mount space for an in-vehicle honeycomb catalyst or the like is limited, and hence mounting the honeycomb catalyst becomes difficult in some cases.
Patent Document 1: JP-A-2003-33664